1. Field of the Invention
The present invention relates to flight control systems for rotorcraft.
2. Description of Related Art
There are many different types of rotorcraft, including: helicopters, tandem rotor helicopters, tiltrotor aircraft, four-rotor tiltrotor aircraft, tilt wing aircraft, and tail sitter aircraft. In all of these rotorcraft, thrust and/or lift is generated by air flowing through a rotor disk formed by rotating rotor blades.
There are three basic flow states for the air flow through the rotor disk of a rotorcraft: (1) the normal working state; (2) the windmill-brake state; and (3) the vortex ring state. These three flow states are typically described in terms of hover induced velocity, which is determined from the momentum theory. The basic premise of momentum theory is that a definite wake field exists far downstream of a hovering rotor. However, when a rotorcraft begins to descend, the assumptions of the momentum theory begin to break down. The normal working state, the windmill-brake state, and the vortex ring state are shown schematically in FIGS. 1A–1C, respectively.
Referring to FIG. 1A in the drawings, the normal working state of a rotorcraft is illustrated schematically. In the normal working state, the air approaches the rotor in the same direction as the induced velocity, i.e., the air flow is downward through the rotor disk. In the normal working state, a definite slip stream exists and the air flow at the rotor disk is always equal to or greater than the induced velocity. The normal working state can exist for rates of climb in the range of zero, i.e., hovering, to infinity.
Referring now to FIG. 1B in the drawings, the windmill-brake state is illustrated schematically. In the windmill-brake state, the air approaches the rotor in the opposite direction of the induced velocity, i.e., the air flow is upward through the rotor disk. In the windmill-brake state, a definite slipstream exists; however, the induced velocity, which  opposes the main air flow, causes a decrease in the velocity of the air flow as the air flow approaches and passes through the rotor disk. This causes the slipstream to expand above the rotor disk. For low rates of descent, the expansion of the slipstream is very large, and substantial recirculation and turbulence are generated.
Referring now to FIG. 1C in the drawings, the vortex ring state is illustrated schematically. In the vortex ring state, the air flow is also downward, because of the large induced velocity; however, the air flow far above the rotor is in an upward direction. In the vortex ring state, the definite slipstream is replaced by large recirculating air flows. The vortex ring state can exist for rates of descent in the range of zero, i.e., hovering, to twice the average induced velocity. At high rates of descent and low horizontal airspeeds, the low wake skew angle and high rotor vertical velocity cause the rotor to re-ingest its wake. Higher collective pitch angles, and thus power, are necessary to maintain constant thrust levels when this occurs. As a result, the vortex ring state is sometimes called “power settling” or “settling with power.”
Existing test data indicates that the vortex ring state is limited to a small range of horizontal airspeeds and vertical velocities proportional to the momentum theory prediction for the rotor's hover induced velocity. The most likely scenario for a rotorcraft to encounter the vortex ring state is during a rapid descent to a landing, especially with an unexpected tailwind component. Operation within these boundaries is highly unsteady and can lead to temporary, and sometimes catastrophic, loss of control. Common precursors to fully developed vortex ring state include thrust fluctuations, erratic rotor flapping, high vibrations, and aural rumbling. 